1. Field of the Invention
This invention relates to a control system for automatic vehicle transmissions.
2. Description of the Related Art
Among others the assignee proposes, in Japanese Laid-Open Patent Application No. Hei 10 (1998)-153,259, a control system for automatic vehicle transmissions. In the proposed system, when the gear is being shifted from the gear (currently engaged) to a destined gear (to be shifted or engaged to next), if the change of the destined gear is required, its change is inhibited or prohibited unless a predetermined condition is met, so as to prevent unpleasant shock which the vehicle occupant would otherwise experience when the destined gear is changed during shift.
More specifically, it is determined in the system whether an input/output rotational speed ratio (indicative of the degree of shift progress which is one of the shift control parameters) has been determined accurately and if not, the change of the destined gear is inhibited. With this, the system aims to suppress the unpleasant shock which is liable to occur when the destined gear is changed during shift.
From a standpoint of improving response, it will be preferable, needless to say, to control shift such that the changed destined gear is immediately established. However, if the change of the destined gear is required when the shift is in progress to a certain degree, for example, when the hydraulic (oil) pressure being supplied to the clutch has reached a level under which the initially destined gear is almost engaged, the destined gear change will cause the slippage time to be longer and hence, to degrade the clutch. In addition, this will generate continuous torque fluctuation and give unpleasant feeling to the vehicle occupant.
Further, the control in change of the destined gear during a jumping shift such as a two-gear downshifting, such as a two-gear downshifting from 4th to 2nd has not been discussed in the prior art and the response to the change of the destined gear at that situation is not always satisfactory.
An object of this invention is therefore to overcome the aforesaid problems and to provide a control system for automatic vehicle transmissions, which can effectively conduct the change of a destined gear when the shift is in progress, in particular, when a two-gear downshifting is in progress, thereby enabling to improve the response to the change of the destined gear and feeling experienced by the vehicle occupant, while preventing the clutch from being degraded.
In order to achieve the objects, there is provided a system for controlling an automatic transmission of a vehicle having an input shaft connected to an internal combustion engine mounted on the vehicle and an output shaft connected to driven wheels of the vehicle, the transmission transmitting input torque, through any of frictional engaging elements, generated by the engine and inputted by the input shaft to the driven wheels by the output shaft, comprising; destined gear retrieving means for retrieving a destined gear to be engaged to in accordance with predetermined shift scheduling map based on detected operating conditions of the vehicle and the engine in response to a shift command; shift mode determining means for determining a shift mode based on the destined gear and a current gear currently engaged, such that the destined gear is established; hydraulic pressure command value determining means for determining hydraulic pressure command value to be supplied to any of the frictional engaging elements based on the determined shift mode; degree of shift progress detecting means for detecting a degree of shift progress in shifting from the current gear to the destined gear; destined gear change possibility determining means for determining, if the shifting from the current gear to the destined gear is downshifting, whether change of the destined gear is possible when the downshifting is in progress, by comparing the degree of shift progress with a predetermined first threshold value; destined gear change permission determining means for determining a permission of the change of the destined gear when the change of the destined gear is determined to be possible; and destined gear determining means for determining the destined gear based on the permission of the change of the destined gear, wherein the shift mode determining means includes: second threshold value comparing means for comparing the degree of shift progress with a predetermined second threshold value, when the downshifting is two-gear downshifting; and determines the shift mode such that the destined gear is established in an upshifting direction.